Wednesday, September 11, 2013

Notes - Shift Ciphers

The following are notes from Introduction to Cryptography with Coding Theory.
  • Shift ciphers are one of the earliest cryptosystems and is attributed to Julius Caesar
    • gaul is divided into three parts
    • he then shifts each letter by 3 places
    • Encryption process is then x -> x + k where k = 3
  • Attackers methodology
    • Ciphertext only
      • If the method is known then there are only 26 possible keys. However there is most likely more than one meaningful message that can be found when using these keys.
      • Frequency analysis to see the shift, e is the most common letter in the English language, find the difference between the most common letter in the message and e
    • Known Plaintext
      • if you know one letter of plaintext the key is broken
    • Chosen Plaintext
      • again inputting even just one letter will give you the key
    • Chosen Ciphertext
      • seeing the output of an input will give you the key

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